Compact sampling device based on wax microfluidics
Graphical abstract
Introduction
Microfluidics show a huge potential for water quality analysis [1], [2], [3]. The development of low-cost, fully-integrated, easy-to-operate and portable sampling and sensing devices is essential to obtain spatial and temporal information on contamination risk. Miniaturized systems are also of special interest due to their suitability to be used on-board the air and on-/under-water drones. Aerial and aquatic drones have emerged as a very powerful tool for automated environmental analysis due to their low-cost technology and their ability to reach areas that can be difficult or hazardous to access by other methods [4].
In recent years, many microfluidic systems for environmental analysis have been developed [5], [6], [7], [8], [9], [10], [11]. Most of them rely on integrated sensors for the analysis of a limited number of chemical species. By contrast, the development of microfluidic systems for sampling and subsequent complete analysis in laboratory is very scarce [12]. Current water collection methods rely on the use of manual or automatic sampling devices deployed in a buoy or autonomous vehicles (vessels, submarines) [13], [14], [15]. Most of these systems are slow, costly, spatially restricted and difficult to deploy. The use of more compact sampling systems on board of small-sized drones has appeared as a promising alternative for water sampling in a timely and cost-effective manner [16]. Nevertheless, collecting a large number of samples on-board of these small devices proves difficult because of the power requirements and size of current sampling systems. Therefore, the development of microfluidic sampling devices is highly desirable.
The most relevant work in the field of microfluidic liquid sampling systems is the aquatic sampler developed by Jonsson et al. [12] for trapping microorganisms from underwater environments. Although not tested in field, the sampler device was designed to collect one sample on-board of a miniaturized submersible explorer. In this sampler, two membrane-type paraffin microvalves were used to keep the collected sample pristine until the analysis.
One of the main difficulties in the development of microfluidic devices for environmental analysis is the lack of reliable, fully-integrated microfluidic valves [17]. Phase-change paraffin valves are particularly attractive for these systems [18]. Paraffin wax has been used either as a propellant of an elastic membrane [12], [19] or as a meltable plug [9], [20], [21], [22] in the development of optically and electrically controlled microfluidic valves. Plug-type valves operated by melting and displacing a wax plug through the microchannels by capillary forces or pressure differences are especially well-suited for their integration in portable and disposable microfluidic devices. Compared to membrane-type paraffin valves, such as those integrated in the deep-sea sampler discussed above [12], plug-type microvalves have a simpler structure and are easier to operate. Paraffin valves are also inherently latched at both the open and closed states, which guarantee low energy consumption, and they can withstand high fluidic pressures. Main limitations of these valves are their slow response times, complex fabrication process and single-use capability.
We have recently developed electrically controlled wax microvalves that overcome all these limitations [23]. These multiple actuation wax valves comprise three heaters for their actuation, and can be easily fabricated as a fully integrated element of wax microfluidic devices. In this work, a new design of wax microvalves actuated by a single electrical heater is described. Based on these new valves, a compact sampler device that enables the collection of up to 14 different water samples is presented.
Section snippets
Sampler design and working principle
As shown in Fig. 1a,b, the sampling device is composed of one 100 μm-thick structured wax layer sandwiched between two glass slides. The Pyrex glass substrate incorporates the microheaters required for valve actuation, while the top borosilicate glass layer includes the holes for fluid access.
The wax pattern comprises a central chamber (17 mm2) surrounded by fourteen 300 μm-wide microchannels. Each of these microchannels connects the central chamber to one collection outlet (1 mm in diameter)
Results and discussion
Fig. 3 shows images of the sampler device in operation. For visualization purposes coloured water samples were collected and the collection tubes were not connected to the system. The pictures taken at different times of the sampling process demonstrated proper opening and closing behaviour of all the wax valves in the chip. It is also clear shown the effective sealing of the system provided by wax. The sampler device is leak-proof to at least 100 kPa and provides a perfect isolation of the
Conclusions
A new ultracompact liquid sampler has been successfully designed and constructed. This wax microfluidics-based device delivers leak- and contamination-free performance that ensures the integrity of the collected samples. The wax microvalves integrated in this sampler demonstrate a reversible open–close behaviour, fast response and extremely low energy consumption. Thanks to the small footprint and simple fabrication process of the developed valves, samplers with capability for larger number of
Acknowledgments
This work was supported by the Spanish Ministry of Economy, Industry and Competitiveness (MEINCOM) under Grant TEC2013-44817-R and by Generalitat de Catalunya (2014SGR1645). This work has made use of the Spanish ICTS Network MICRONANOFABS partially supported by MEINCOM.
Dra. María Díaz-González obtained her PhD in Chemistry from the University of Oviedo (Spain) in 2006. From 2006–2009 she held a posdoctoral position at the Polytechnic University of Catalonia (Spain). Since 2010 she is a postdoctoral researcher at the IMB-CNM (CSIC). Her area of expertise involves the development of advanced analytical microsystems integrating electrochemical sensing devices.
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Dra. María Díaz-González obtained her PhD in Chemistry from the University of Oviedo (Spain) in 2006. From 2006–2009 she held a posdoctoral position at the Polytechnic University of Catalonia (Spain). Since 2010 she is a postdoctoral researcher at the IMB-CNM (CSIC). Her area of expertise involves the development of advanced analytical microsystems integrating electrochemical sensing devices.
Dr. Antonio Baldi BS degree on Telecommunication Engineering (U. Politècnica de Catalunya, 1996), and Ph.D. on Electronics Engineering (U. Autònoma de Barcelona, 2001). From 2001–2003 he was at the U. of Minnesota, working in bioMEMS. In 2003 he joined the Chemical Transducers Group at the IMB-CNM (CSIC). His current research is focused on lab-on-a-chip devices for chemical sensing and biodetection.
Dr. César Fernández-Sánchez, PhD in Chemistry. He holds a permanent position as a Senior Researcher at IMB-CNM. He is currently the Head of the Chemical Transducers Group at IMB-CNM (CSIC). He has actively worked on the development of analytical microsystems integrating electrochemical sensors based on different transduction modes and (bio)chemical receptors. His research interests also include the biofunctionalisation of substrates and the application of nanomaterials and organic-inorganic polymeric materials for the development of chemical sensors.